1. Field of Invention
This invention relates to dispersion slope compensation in optical transmission systems and more particularly to compensation in wavelength division multiplex systems (WDM) having one or more branches.
2. Description of Related Art
In a conventional WDM transmission system such as is shown in FIG. 1 a transmitter 10 is arranged to provide on the trunk fiber 12 different traffic signals each on a different wavelength which wavelength is intended for receipt by a specific receiver which may be located at the end of a branch from the trunk. Such systems normally employ optical amplifiers/repeaters 14 at spaced locations along the trunk to compensate for signal attenuation with distance along the trunk. Such systems employ dispersion shifted optical fiber (DSF) with the channels located in the negative dispersion regime (with the channel wavelengths shorter than the wavelength of minimum dispersion, .lambda..sub.0, of the fiber). One method, known to us, of compensating for dispersion occurring on the trunk is for the net dispersion to be periodically equalised using non-dispersion shifted fiber (NDSF) 16 with a .lambda..sub.0 of around 1300 nm (positive dispersion regime). The system can only be equalised to a net dispersion zero at one particular wavelength, without splitting the channels and individually equalising them (very complicated). Consequently the other channels will accumulate additional dispersion depending upon the wavelength offset from the net .lambda..sub.0 and the dispersion slope of the transmission fibre. This differential dispersion is not reset by the equalisation procedure. The effect of dispersion on the longest and shortest wavelengths at two locations along the trunk are illustrated by FIGS. 1a and 1b whilst the effect of compensation is illustrated by FIG. 1c.
U.S. Pat. No. 5,224,183 describes a WDM signal compensation system in which dispersion is under compensated and the remaining compensation is carried out via separate means per channel either at the transmitter or receiver end. This adds considerably to the complexity of the system.